Sales Reality Check
Is your Cook County home assessed above what homes like it actually sell for? This runs a full IAAO sales-ratio study against recorded arm's-length sales — the same method an assessor uses to check their own work.
Free. Cook County only, for now — it needs recorded sales joined to the assessment roll, and Cook is the county that publishes both.
What this actually does
Illinois lets you challenge an assessment on two independent grounds. Most tools — including the rest of this site — argue uniformity: that your assessment per square foot is out of line with comparable properties. This page argues the other one, market value: that your property is assessed above what properties like yours actually sell for.
It runs a sales-ratio study, the method assessors use on themselves and the one the IAAO publishes a standard for. For every arm's-length sale nearby it computes assessed value ÷ sale price. The median of those ratios is the level at which your neighbourhood is really assessed. Cook's statutory level for residential property is 10%; what the sales say is often something else.
Which sales count, and which do not
This is where a ratio study is won or lost, so it is stated rather than hidden:
- Arm's-length transfers only. Warranty, special warranty and trustee deeds. A foreclosure, a judicial sale or an estate winding up is a real transaction and none of them is evidence of what a willing buyer pays a willing seller.
- Nothing under $25,000, which filters transfers between relatives and clerical corrections rather than houses.
- Last 72 months, with recent sales weighted more heavily — a two-year-old sale counts half as much as one from last month.
- Same major property class. A house is not compared to a condo.
- Outliers trimmed on the inter-quartile rule, and the number removed is reported.
Three numbers, and what each one means
| Level of assessment | The median ratio. If it reads 11.3% while the statutory level is 10%, the whole neighbourhood is assessed above market. |
|---|---|
| COD | Coefficient of Dispersion — how much assessments vary between similar homes. IAAO calls 10–15 acceptable for residential. Above that, the roll is being applied unevenly, which is itself grounds for a uniformity appeal. |
| PRD | Price-Related Differential. Above 1.03 the roll is regressive: cheaper homes carry a higher share of their value than expensive ones. It is the least visible inequity in mass appraisal and the one that hurts people least able to hire somebody to notice it. |
Where this can be wrong, stated plainly. The market estimate for your property is built from the price per square foot of homes that sold. Homes that sell are newer and better presented than homes that do not, so that figure runs high — which makes this tool conservative about finding over-assessment. It will miss some real cases rather than invent one. A verdict needs at least 12 comparable sales to appear at all and 30 before it is called confident.
ParcelProof is not a law firm, does not file on your behalf, and guarantees no outcome. Confirm every figure against your own assessment notice before relying on it.